Lightweight mirrors have great utility in optical systems that require mirrors which are resistant to distortion caused by gravitational sag effects. Lightweight mirrors are also advantages in airborne and space-based optical systems wherein a reduction in overall weight of the optical system in desired. Lightweight mirrors are also advantageous in cryogenic optical systems wherein a reduction in the thermal mass of the mirror facilitates the cooling of the mirror and the maintenance of the mirror at a desired low temperature.
It has been known in the art to fabricate lightweight mirrors over a core or substrate which is comprised of webs made from a variety of materials, such as fused silica, beryllium (Be) and cast silicon carbide (SiC).
The core typically separates two relatively thin facesheets, at least one of the facesheets being polished to provide the mirror face. One such lightweight mirror of the prior art is described in a journal article entitled "Fabricated Lightweight Metal Optic Substrates", S.P.I.E., vol. 131, page 85 (1978), by L. A. Grant.
Such mirrors of the prior art typically comprise a system of webs which serve as the mirror substrate. The webs are often arranged as square cell cores upon which the facesheet material is bonded. Due to the nature of the square cell core substrate, the facesheets must have a thickness which is sufficient to maintain the facesheet dimensional stability over those regions of the core defined by the large voids within the square cells. This requisite minimum thicknss of the facesheet tends to make the facesheets relatively thick, thereby adding to the weight and thermal mass of the mirror.